ISO Sensitivity / Noise levels

ISO equivalence on a digital camera is the ability to increase the sensitivity of the sensor to enable faster shutter speeds and/or better performance in low light. The way this works in a digital camera is by "turning up the volume" on the CCD's signal amplifiers, nothing is without its price however and doing so also amplifies any noise that may be present and often affects colour saturation.

Just like the S304 the S404 provides four selectable ISO sensitivites. However, on the S404 the range of sensitivities has been shifted towards slower speeds. So, instead of ISO 100 - 800 (S304) we now have a selection of sensitivities of ISO 64 - 100.

Our noise testing method involves shooting a colour patch chart (a GretagMacBeth ColorChecker) at the full range of ISO sensitivities.

Each of the comparison crops below are made up of a 180 x 80 crop of patches 19, 22 and 24 of the ColorChecker chart (as indicated above). This helps to give a better impression of noise at different light levels within an image. Directly below this you will find the average standard deviation of these three patches, this is a quantifiable measurement of noise.

Both cameras were set to automatic white balance, default internal processing parameters. Both cameras set to aperture priority, the S404 only provides two apertures at any focal length, the closest match I could produce was F3.6 on the S404 and F3.5 on the G2.

Minolta DiMAGE S404 vs Canon PowerShot G2 in low light

Camera	Minolta DiMAGE S404	Canon PowerShot G2
Sensitivity	ISO 64	ISO 50
Exposure	0.7 s, F3.6	1/2 s, F3.5
Patch crops  ISO 64 / 50
Standard dev. avg.	1.41	1.38
Camera	Minolta DiMAGE S404	Canon PowerShot G2
Sensitivity	ISO 100	ISO 100
Exposure	1/2 s, F3.6	1/4 s, F3.5
Patch crops  ISO 100
Standard dev. avg.	1.57	2.04
Camera	Minolta DiMAGE S404	Canon PowerShot G2
Sensitivity	ISO 200	ISO 200
Exposure	1/4 s, F3.6	1/8 s, F3.5
Patch crops  ISO 200
Standard dev. avg.	2.85	3.16
Camera	Minolta DiMAGE S404	Canon PowerShot G2
Sensitivity	ISO 400	ISO 400
Exposure	1/8 s, F3.6	1/20 s, F3.5
Patch crops  ISO 400
Standard dev. avg.	2.85	5.36

The first thing to note is the large difference between the supposed sensitivity as it is selected on the camera and the actual sensitivity. At ISO 200 the S404 has a metered exposure of 1/4 sec (F3.6) where as the G2 has an exposure of 1/8 sec (F3.5) a whole STOP different.

I measured the available light (using a Sekonic light meter) as 5.0 EV, this makes the S404 closer to the correct sensitivity and the G2 almost a stop more sensitive than the selected speed. Thus G2 ISO 100 = approx. ISO 200, G2 ISO 200 = approx. ISO 400, etc.

This of course doesn't work to the G2's favour, in this test we are comparing noise levels at each sensitivity (as selected on the camera). Below is a graph of measured noise at each selected sensitivity.

White Balance

The S404's automatic white balance worked well in direct sunlight although it did have a tendancy to produce a slight blue cast in cloudy or shade conditions. That said the preset Cloudy, Daylight and Incandescent settings worked very well. The manual preset option produced consistent and accurate white balance.

Images re-saved to sRGB colour space [info]

Outdoors, Auto	Outdoors, Cloudy (or Daylight)	Outdoors, Manual
Incandescent, Auto	Incandescent, Incandescent	Incandescent, Manual
Fluorescent, Auto	Fluorescent, Fluorescent	Fluorescent, Manual

First word indicates the light in whcih the shot was taken, the second indicates the selected white balance mode.

Low Light Focus

In a new addition to our reviews we'll now be measuring the minimum amount of light under which the camera can still focus. The focus target is our lens distortion test chart (shown here on the right), camera is positioned exactly 2 m (6.6 ft) away.

Light levels are gradually dropped until the camera can no longer focus. This is carried out at both wide angle and telephoto zoom positions (as more light reaches the focusing systems with a larger aperture).

This test target is the optimum type of subject for most "contrast detect" AF systems (as it has a vertical line at its center), you should consider the results below the best you could expect to achieve.

Light intensity (Lux) = 2.5 x 2^EV (@ ISO 100), 10.76391 Lux = 1 foot-candle (fc)

As with the S304 the S404 manages to still focus at fairly low light levels. Despite it's F3.0 maximum aperture at wide angle the lowest light focus of 1.5 EV is fairly respectable.